A Multiscale Model to Study the Mechanical Properties of the Graphene, Boron Nitride and Silicon Carbide Hexagonal Nanosheets

Background: It is very important to precisely comprehend nanosheet’s mechanical properties for their future application, and the continuum-based methods play a vital role in this research domain. But, most of continuum models doesn’t provide a systematical theory, and just display certain property of nanostructures. The Cauchy-Born rule provides an alternative multiscale method, the resulted model is not only less accurate, and but also doesn’t describe the bending effect. Methods: A nanosheet is viewed as a higher-order gradient continuum planar sheet, and the strain energy density is thus a function of both the first- and second-order deformation gradient. The higher-order Cauchy-Born rule is used to approximate the bond vectors in the representative cell, the multiscale model is established by minimizing the cell energy, and the structural and mechanical properties are thus obtained. Results: The obtained bond lengths are respectively 0.14507nm, 0.14489nm, 0.1816nm for the graphene, boron nitride and silicon carbide hexagonal nanosheets. The elastic constants, including Young’s modulus, shear modulus, Poisson’s ratio and bending rigidity, are calculated by analyzing the physical meaning of the first- and second-order strain gradients. The developed model can also be used to study the nonlinear behavior of nanosheets under some simple loading situations, such as the uniform tension, torsion and bending. The stress-strain relationship of nanosheets is presented for the uniform tension/compression, and the three types of nannosheets exhibit better compressive resistance far greater than tensile resistance. Conclusion: A reasonable multiscale model is established for the nanosheets by using the higher-order Cauchy-Born rule that provides a good interlinking between the microscale and continuum descriptions. It is proved that all three types of nannosheets shows the isotropic mechanical property. The current model can be used to establish a global nonlinear numerical modeling method in which the bending rigidity is the basic elastic constants same as the elastic modulus and Poisson’s ratio.

Dynamic Characteristics and Stability of Flexible Riser Under Consideration of Non-Uniform Tension and Internal Flow

As oil and gas industry is developing towards deeper ocean area, the length and flexibility of ocean risers become larger, which may induce larger-amplitude displacement of flexible riser response due to lower structural stiffness against environmental and operational loads. Moreover, suffering not only the external fluid loads coming from environmental ocean wave and current, these risers also convey internal flow. In other words, the dynamic characteristics and response of the flow-conveying riser face great challenge, such as bucking, divergence and flutter, because of the fluid-solid coupling of the internal hydrodynamics and riser structural dynamics. In this study the dynamic characteristics and stability of a flexible riser, under consideration of its internal flow and, particularly, non-uniform axial tension, are examined through our FEM numerical simulations. First, the governing equations and FEM models of a flexible riser with axially-varying tension and internal flow are developed. Then the dynamic characteristics, including the coupled frequency and modal shape, are presented, as considering the speed of internal speed changes. At last, the dynamic response and corresponding stability behaviors are discussed and compared with the cases of riser with uniform tension. Our FEM results show that the stability and response are quite different from riser with uniform tension. And, the time-spatial evolution of riser displacement exhibit a strong wave propagation phenomenon where travelling wave are observed.

Actin turnover ensures uniform tension distribution during cytokinetic actomyosin ring contraction

In many eukaryotes, cytokinesis is facilitated by the contraction of an actomyosin ring (AMR). The exact mechanisms that lead to this contractility are unknown, although some models posit that actin turnover in the AMR is essential. The effect of reduced actin dynamics during AMR formation has been well studied in Schizosaccharomyces pombe; however, the corresponding effects on AMR contraction are not well understood. By using mutants of the fission yeast actin severing protein Adf1, we observed that contracting AMRs display a “peeling” phenotype, where bundles of actin and myosin peel off from one side of the AMR, and are pulled across to the opposite side. This occurs multiple times during cytokinesis and is dependent on the activity of myosins Myo2, Myp2, and Myo51. We found that the distribution of Myo2 in the AMR anticorrelates with the location of peeling events, suggesting that peeling is caused by a nonuniform tension distribution around the AMR, and that one of the roles of actin turnover is to maintain a uniform tension distribution around the AMR.